The immune competence of green sea turtles (Chelonia mydas) with fibropapillomatosis was assessed using in vitro techniques to measure lymphocyte proliferation in response to mitogens. In comparison with captive, healthy green sea turtles, those afflicted with fibropapillomas demonstrated diminished proliferation with Concanavalin A, phytohemagglutinin (T-cell mitogens), and lipopolysaccharide (B-cell mitogen). Also, markedly decreased proliferative responses to the lymphocyte polyclonal stimulator combination of ionomycin and phorbol myristate acetate were observed. Total circulating white blood cell counts were not statistically different between the two groups, although an overall decrease in lymphocyte number was observed in the papilloma group. The albumin/globulin ratio was decreased in the papilloma group because of decreased albumin and increased gamma globulins.